1. Field of the Invention
This invention relates to pressurized operation of fuel cells including an associated gas turbine generator within the pressure vessel.
2. Background Information
Fuel cell based, electrical generator apparatus utilizing solid oxide electrolyte fuel cells ("SOFC") arranged within a housing and surrounded by insulation are well known for tubular, flat plate, and "corrugated" SOFC. The tubular type fuel cells can comprise an open or closed ended, axially elongated, ceramic tube air electrode material, which may or may not be deposited on a ceramic support tube, completely covered by thin film ceramic, solid electrolyte material, except for a thin, axially elongated, interconnection material. The electrolyte layer is covered by cermet fuel electrode material. Other tubular designs can be configured with multiple cells per single tube, cathode supported, anode supported, support tube (ceramic or metallic) supported and with the exterior surface being either the anode or the cathode. The flat plate type fuel cells can comprise a flat array of electrolyte and interconnect walls where electrolyte walls contain thin, flat layers of cathode and anode materials sandwiching an electrolyte. The "corrugated" plate type fuel cells can comprise a triangular or corrugated honeycomb array of active anode, cathode, electrolyte and interconnect materials. Other fuel cells not having a solid electrolyte, such molten carbonate fuel cells are also well known, and can be utilized in this invention.
Development studies of SOFC power plant systems have indicated the desirability of pressurized operation, as taught in "Solid Oxide Fuel Cell . . . Power Generation of the Next Decade", Westinghouse Electric Corporation, Brochure 1992. This would permit operation with a coal gasifier as the fuel supply and/or use of a gas turbine generator as a bottoming cycle. Integration is commercially possible because of the closely matched thermodynamic conditions of the SOFC module output exhaust flow and the gas turbine inlet flow.
A variety of fuel cells used in power plant systems are described in the literature. For example, in U.S. Pat. No. 5,413,879 (Domeracki et al.) a SOFC is integrated into a gas turbine system. There, pre-heated, compressed air is supplied to a SOFC along with fuel, to produce electric power and a hot gas, which gas is further heated by combustion of unreacted fuel and oxygen remaining in the hot gas. This higher temperature gas is directed to a topping combustor that is supplied with a second stream of fuel, to produce a still further heated gas that is then expanded in a turbine.
A variety of fuel cell types, in various system configurations is also described by B. R. Gilbert et al. in "Fuel Cells Make Their CPI Moves", in Chemical Engineering, August 1995, pp. 92-96. Specifically, a conceptual design of a 1 MW commercial unit shows two molten carbonate fuel cell stacks and two associated reformers enclosed within a horizontal cylindrical vessel. The same concept is also reviewed by S. E. Keuhn in "Molten-Carbonate Fuel Cell Demonstrates its Commercial Readiness", Power Engineering, March 1995, p. 16.
U.S. Pat. Nos. 5,573,867 and 4,750,278 (Zafred et al. and Gillett et al. respectively) describe SOFC systems in pressure vessels. However, all these systems separate the fuel cells, in their pressure vessel, from the combustors, invertors, plant control systems, and turbine generators. Present pressurized solid oxide fuel cell generator/ gas turbine systems ("PFC/GT") consist of one or more of tubular fuel cells arranged in a pressure vessel together with a set of isolation and stack bypass valves and associated piping. The gas turbine's expander and compressor sections are located on a skid external to the generator pressure vessel. The expander driven compressor supplies process air to the fuel cell stacks through a recuperator and an SOFC generator inlet check valve. The SOFC's hot pressurized exhaust stream is expanded through the gas turbine's expander and is then used to preheat the air in the recuperator. Much of this is taught and shown in the Domeracki et al '879 patent.
There are several concerns with using small gas turbines (micro-turbines) as the air source for the fuel cell stack. Reliability of the gas turbine is a concern, as outages imply depressurization events and thermal cycles for the fuel cell. Further, the ability of the gas turbine to maintain a steady flow of air and maintain a steady pressure within the fuel cell in the event of fluctuating turbine inlet temperature is in question. With the cell stack(s) operating at pressure, any significant reduction in the air side pressure due to gas turbine speed fluctuations could result in the unwanted back flow of fuel gas in the fuel cells. Experiments have shown that even short exposures of the air electrodes to fuel can lead to failure of the fuel cells. Clearly, a major mechanical failure of the compressor or expander could lead to the depressurization of the vessel through the air inlet side with subsequent stack damage. Further, should depressurization occur through the fuel cell exhaust, then the gas turbine's expander can be subject to temperatures in excess of its capacity. To safeguard against these events, the SOFC/GT system typically has three valves incorporated into the piping. An SOFC generator inlet check valve is provided to prevent the unwanted back flow of fuel gas through the air inlet piping during a pressure excursion. A bypass line and valve may be provided around the stack(s) to both modulate the airflow to the stack and to aid in starting the turbine. Finally a valve is placed in the SOFC generator exhaust piping to control the depressurization of the SOFC generator through the gas turbine expander. These valves, carrying hot gases and typically located in the hot surroundings within the pressure vessel are very expensive and difficult to maintain. The cost of this equipment may be excessive, making eventual commercialization of this approach questionable. What is needed is an alternate arrangement of the above mentioned components to increase the reliability and reduce the cost of the total system.